Bandits in northwestern Nigeria's Zamfara state have killed 33 people they had kidnapped in February despite receiving $33,250 in ransom, local officials and residents said on Monday.

Zamfara and other states in central and northwestern Nigeria have, for years, been terrorised by criminal gangs who carry out deadly raids, kidnappings and arson attacks.

"The bandits released only 18 of the 51 people they kidnapped from our village and when we asked (the freed hostages) about the 33 others they said they had been killed by their captors," Kasimu Ibrahima, a resident of Banga village, told AFP.

Local political administrator Mannir Haidara confirmed that some captives had been killed but did not give a number.

Herder-farmer conflict

The violence initially stemmed from conflicting claims for land and water rights between herders and farmers but morphed into organised crime, with gangs taking control of neglected rural communities with little or no government presence.

In February, bandits stormed into Banga village in the Kaura Namoda district on motorcycles and abducted 51 people, including three pregnant women, after killing two others, residents said.

The locals then paid the ransom in two instalments.

But on Friday 18 captives returned to Banga and reported the killing of 33 fellow captives by their abductors.

"They (bandits) took the money we worked hard to raise, killed 33 of the captives and sent the remaining 18 back to us," Altine Bawa, another resident, said.